As shown in FIG. 2, a double-tube type heat exchanger having a cylindrical inner tube 101 and an outer tube 102 so surrounding the peripheral surface of the inner tube 101 as to enclose it is known. A port 105 at one end of the outer tube 102 of a double-tube type heat exchanger 103 is connected to an outflow end 107A of a rectification circuit 107, while a port 106 at the other end of the outer tube 102 is connected to an inflow end 107B of the rectification circuit 107 via a main electromotive-expansion valve 108. The outflow end 107A is connected to an hole 111 of the inner tube 101 on the upstream side thereof via a bypass electromotive-expansion valve 112. An hole 113 of the inner tube 101 on the downstream side thereof is connected to a bypass pipe 115.
The rectification circuit 107 has four check valves 121, 122, 123, and 124 connected in a forward direction from the inflow end 107B to the outflow end 107A. A connection pipe 107C connecting the check valves 121 and 123 to each other and a connection pipe 107D connecting the check valves 122 and 124 to each other serve as the connection pipes connected to a main-flow circuit. A thermostat 119 installed on a bypass pipe 114 detects the temperature of a bypass-flow refrigerant. Temperature information detected by the thermostat 119 is used to control an open degree of the bypass electromotive-expansion valve 112.
As shown in FIG. 3, a gas injection circuit can be constructed by connecting the bypass pipe 115 to an intermediate-pressure position of a compressor 116 and by connecting connection pipes 107C and 107D to an outdoor heat exchanger 201 and an indoor heat exchanger 202, respectively. According to the gas injection circuit, in a cooling time, a refrigerant discharged from the outdoor heat exchanger 201 serving as a condenser is expanded by the bypass electromotive-expansion valve 112 and introduced into the inner tube 101. After the refrigerant is heated by a main-flow refrigerant inside the outer tube 102, it can be injected to the intermediate-pressure position of the compressor 116 via the bypass pipe 115. In a heating time, a refrigerant discharged from the indoor heat exchanger 202 serving as a condenser is heated by a refrigerant inside the outer tube 102 after the refrigerant passes through the bypass electromotive-expansion valve 112 and the inner tube 101. Then, the refrigerant can be injected to the intermediate-pressure position of the compressor 116 via the bypass pipe 115.
As shown in FIG. 4, by connecting the bypass pipe 115 to an intake side of the compressor 116 and connecting the connection pipes 107C and 107D to the outdoor heat exchanger 201 and the indoor heat exchanger 202, respectively, a super-cooling circuit can be constructed. According to the super-cooling circuit, in a cooling time, a refrigerant discharged from the outdoor heat exchanger 201 is expanded by the bypass expansion valve 112 and introduced into the inner tube 101. After a main-flow refrigerant inside the outer tube 102 is super-cooled, the refrigerant can be returned to the intake side of the compressor 116 via the bypass pipe 115. In a heating time, a refrigerant discharged from the indoor heat exchanger 202 is expanded by the bypass electromotive-expansion valve 112 and introduced into the inner tube 101. After the main-flow refrigerant inside the outer tube 102 is super-cooled, the refrigerant can be returned to the intake side of the compressor 116 via the bypass pipe 115.
However, according to the conventional double-tube type heat exchanger 103, in order to construct the gas injection circuit or the super-cooling circuit, a pressure-reducing mechanism, namely, the bypass electromotive-expansion valve 112 is required as described above. The bypass electromotive-expansion valve 112 causes the construction of the conventional double-tube type heat exchanger 103 to be complicated and its cost to increase.